In humans, CD8+ cells play a vital role in the immune system's ability to defend against potentially harmful foreign entities, such as bacteria and viruses [1]. CD8+ cells circulate in the blood and possess on their surface the CD8 protein. When necessary, these cells are converted into cytotoxic cells (i.e. cell-killing cells) which proceed to destroy foreign cells, viruses, and other harmful pathogens present in the subject [2]. Because of CD8+ cells' effective role in host defense, they hold great potential in isolated form as therapeutics for treating disorders such as viral infections and malignancies [3].
In the past, purification of human CD8+ cells has been achieved by negative selection. Specifically, peripheral blood mononuclear cells (“PBMC's”) are incubated with a cocktail of monoclonal antibodies specific for non-CD8 sub-populations. These sub-populations include, for example, B-cells, CD4+ cells, NK cells, macrophages and neutrophils, and each contains specific, non-CD8 “markers”. The sub-populations are then removed from the resulting antibody cocktail using magnetic beads [4]. This technique has certain major disadvantages. The first is that several monoclonal antibodies are required for removing non-CD8+ cells. The second is that the resulting CD8+ population suffers from contamination from non-CD8+ cells that possess relatively low levels of non-CD8 markers. Finally, when a magnetic separation procedure is used to remove all non-CD8+ cells, a large number of magnetic beads are needed.